1. Field of the Invention
This invention relates to plastic knitted netting and its use in wrapping loads on pallets and in wrapping agricultural loads such as cylindrical bales of hay. Further, this invention relates to a method of making thermoplastic ribbons used to knit the netting so as to achieve the surprising result of excellent elasticity, elongation and residual elasticity.
2. Description of Related Art
Wrapping pallet loads and agricultural loads with plastic film and with plastic netting to hold and stabilize the loads has become a widespread practice. The plastic netting which has heretofore been used has been either extruded netting or knitted netting, such as Raschel netting. Some manufacturers of extruded netting have discovered that it is desirable to manufacture a netting having a high degree of stretch before tensile failure and having a high degree of elasticity. In the past, users of netting who have wanted high strength in their netting have had to use extruded netting. Extruded netting has had greater strength than knitted netting due to the much greater amount of material (i.e., greater cross-sectional area) contained in each strand of netting. With the recent awareness that extruded netting could also be made elastic as well as strong, there has been more interest in the use of extruded netting than in the past.
Knitted plastic netting is made of very thin ribbons of thermoplastic material in order to allow it to be knitted; and because of the thinness of the ribbons, knitted netting has not been considered as useful as extruded netting for heavy loads where tensile strength is important. Because of this perception, manufacturers of knitted netting have always attempted to make their netting as strong as possible while still using thin ribbons of thermoplastic in order to keep cost and weight as low as possible.
Whether one is using extruded netting or knitted netting, it is desirable to have a netting which is both strong and elastic. Strength is obviously an important physical characteristic in order to avoid having the netting break and causing the load to spill. Elasticity in a netting is a desirable characteristic because it provides continued tension to the load after wrapping and after the load may have experienced settling and a reduction in volume. A load wrapped by a netting having insufficient elasticity will become loose and unstable. Even with loads which will not settle or reduce in volume, it is undesirable to use a non-elastic netting because tightening the netting sufficiently to hold the load can sometimes damage the contents. Whereas, use of an elastic netting can provide sufficient load-holding tension without damage to the contents.
Although extruded netting is advantageous to use in some applications, it has many undesirable characteristics when compared to knitted netting. One, much less extruded netting can be wound on the same diameter spool compared to a spool of knitted netting. This disadvantage is important when one realizes how frequently rolls have to be changed on a pallet wrapping machine or how much storage space is needed to store the necessary inventory of netting. The same amount of knitted netting of this invention takes one-third the space of some conventional extruded netting.
Also, extruded netting creates great disposal problems. Because of the integral joining of the strands in extruded netting, the structure is very inflexible and is very difficult, if not impossible, to compact into a small space; and when it is done, the netting tends to spring open. The knitted netting of this invention is very easy to dispose of and can be easily gathered into a small volume for disposal.
It would be very desirable to combine the benefits of extruded netting and knitted netting in one netting product; however, in the past there has never been a way to combine the features of strength, stretchability and elasticity in one netting. This has been because the techniques used to strengthen a thermoplastic ribbon were the very techniques used to make a ribbon inelastic, and the techniques used to make a thermoplastic ribbon elastic resulted in a weaker ribbon.